Well the 450 shocks will fit if that's what you have. When I was throwing the arms on a while back I had a set of 450 shocks that I bolted on just to see whats up. It will just sit a bit lower but rideable no doubt if thats all you have to work with for now.
Here is one of the pics of one installed when messing around in the shop. http://i27.photobucket.com/albums/c164/savage_420/DSC00150.jpg
I run some 18" Gen 2 Fox Float X Evols with these arms as I said above. Love them.
One thing I found with these floats, is that take your time and set them up right. Write down what settings you set the hi and low comp and rebound and especially how much pressure you put in the main air chambers and evol chambers. I've played with the air pressures, hi/low comp and rebound a shit ton now and everytime its that much better for every type of riding. Crazy adjustable shocks, can totally change them up in a minute or 2 out on the trail or at the track with the lil pump.
Good times in the hills!!!
Damn that's lots of typing with sausage fingers on my phone.....